Digital systems often comprise a number of functional blocks connected by one or more bus structures. An example of a functional block is special purpose accelerator, such as a vector processor or image processor. Functional blocks may be connected using a number of different bus architectures, such as AMBA (Advanced Micro-controller Bus Architecture). AMBA is an open standard, on-chip bus specification that details a strategy for the interconnection and management of functional blocks that make up a System on-Chip (SoC). Special purpose accelerator blocks outside of a processor core often do not have access to the memory address translation functions provided by the processor code, or along the datapath between the code and the main bus. Such devices are usually restricted to “physical addresses” (addresses in a physical memory space), forcing programming model changes to make them useful.
When a system is initialized, large “worst case” contiguous memory regions must be allocated for use by these accelerators. This prevents memory from being allocated at run-time and does not allow non-contiguous memory to be used.
Sun Workstations, circa 1990, used a feature called “DVMA” which allocated some virtual space for device use. However, this feature worked with a restricted address range and its primary intent was to reduce the need for cache flushing in a virtual cache environment.
Special purpose accelerators are used in many applications, including cellular telephones, digital cameras, PDAs and automotive collision avoidance devices.